There are four basic types of bridge structures: rigid beam, arch, suspension, and cantilever.
The rigid beam bridge is the simplest and most common type of bridge. It covers a relatively short span. A rigid beam bridge consists of straight vertical slabs, or girders, that are embedded in the ground and on which the roadbed is carried. The load (weight) is supported from below by the girders.
The foundation of the arch bridge is an arched, compressed slab (shaped like the top half of a circle) that thrusts outward against supports at each end. The roadway and load rests primarily on the arch. There are vertical girders positioned at given intervals throughout the length of the bridge, which also provide support.
In the suspension bridge, the roadway hangs from steel cables. The entire structure is supported by two or more towers that extend far above the roadway and are anchored into the ground. The bulk of the load rests on cables, which are anchored into the banks. This type of bridge can span a great distance.
A cantilever bridge operates on the same principle as the rigid beam bridge; however, the cantilever can span greater distances than the rigid beam. It has two arms, each of which is, or could be, freestanding. The arms are attached to vertical supports, which are anchored into the ground. The arms project into, and connect at, a central truss (structural frame).
Sources: Finniston, Monty, ed. Oxford Illustrated Encyclopedia of Invention and Technology, vol.6, 48, 57; The Illustrated Science and Invention Encyclopedia, vol 3., pp. 353, 357-59; Stephens, John H. The Guinness Book of Structures, pp. 11-15; Travers, Bridget, ed. World of Invention, pp. 98-99.